You can use pictures from free sites but if you are going to post them under photography or photochallenge you need to let people know you didnt take them.

Creative Commons: If you are posting content under a Creative Commons license, please attribute and link according to the specific license. If you are posting content under CC0 or Public Domain please consider noting that at the end of your post.

Not indicating that the content you copy/paste is not your original work could be seen as plagiarism.